The invention relates to the recycling of powder coating waste.
The preparation of powder coatings is mostly carried out by extruding the powder coating ready formulated by dry mixing of all the required components (powder coating fresh material) in the form of a paste-like melt, cooling off of the melt, coarse comminution into so-called powder coating chips, fine milling (grinding) and subsequent sieving and classifying to desired grain fineness (powder coating classification). Powder coating material that is too coarse-grained (high grain) on sieving can again be fine-milled. Powder coating material that is too fine-grained (low grain) represents waste.
Powder coating waste also arises in the form of powder coating overspray during the powder coating application.
Powder coating waste should if possible be recovered as powder coating to be employed for the original use, not only because of environmental considerations, but also because powder coatings are precious materials.
Various methods which allow recycling of powder coating waste to be used for the original purpose are described in the patent literature.
DE-A-40 28 567 thus describes direct recycling of powder coating overspray by addition to the powder coating fresh material or during extrusion. The process requires considerable expenditure during extrusion of the finely divided material.
In WO 96/15891 pressing of powder coating waste into friable tablets, which are then extruded together with powder coating fresh material, is described.
It is known from EP-A-0-683 199 to form a layer of powder coating waste, to expose the latter to heat, wherein the powder coating particles coalesce without degradation or cross-linking of the powder coating. IR-radiators can serve as the heat source. The coalesced powder coating particles are then again extruded alone or together with powder coating fresh material.
The processes known from DE-A-40 28 567, WO 96/15891 and EP-A-0-683 199 include a re-extrusion of the powder coating waste. This is linked to the danger of a too high thermal load.
WO 98/33848 describes the agglomeration of finely divided powder coating residues under the influence of pressure. The agglomerates can be returned to the powder coating preparation process.
WO 99/23068 and WO 99/23176 describe compacting of powder coating waste by applying pressure. The compacted powder coating waste can be ground together with fresh powder coating material that is to be ground.
The processes known from WO 98/33848, WO 99/23068 and WO 99/23176 require the application of pressure by suitable pressing devices.
WO 00/15705 describes the heating of powder coating fine grain without melting or cross-linking it completely. The powder coating fine grain thereby becomes sufficiently sticky and forms an agglomerated mass, which is treated further after cooling off in order to prepare powder coating particles therefrom having the desired particle size. The agglomerated mass is a powder coating material which is not convenient in operation.
EP-A-0 982 380 describes the preparation of powder coating particles having a particle size suited for application by agglomeration of powder coating fine grain by heat and preferably with the use of auxiliary products promoting the agglomeration. The process is extremely difficult to control due to the high requirements concerning the particle size distribution of the powder coating suited for application.
An object of the invention is to provide a simple process for recycling powder coating waste. It should be possible to carry out the process without pressure, i. e. without application of pressing forces onto the powder coating waste. The powder coating material to be recycled should be subjected as little as possible to heat. There should be no obligation to add auxiliary products, as these would necessarily bring about a change in the composition of the powder coating or would have to be removed.
In its broadest aspect, the invention provides a process for recycling powder coating waste comprising the steps of:
intimately contacting powder coating waste to powder coating chips; and
heating the combination to bond the powder coating waste to the powder coating chips.
This and other aspects of the invention will become apparent on a further reading of the specification.